Automotive wheel balancing apparatus and method

ABSTRACT

Dynamic wheel balancing is achieved at low (1-3 RPS) rotating velocity by mounting the wheel for rotation on a frame which is pivotally supported for any rotation caused by wheel unbalance. A transducer senses any frame movement and electrical signals corresponding to the maximum amplitude is produced by the transducer which signals are converted to indications of the amount of weight needed to balance the wheel and the location thereof.

This application is a continuation of application Ser. No. 07/139,124filed Dec. 23, 1987, now U.S. Pat. No. 5,025,662; which is acontinuation of application Ser. No. 06/502,986 filed Jun. 10, 1983,abandoned; and is a substitute of application Ser. No. 06/846,782 filedApr. 1, 1986, abandoned; which is a continuation of application Ser. No.06,502,986.

BACKGROUND OF THE INVENTION

For as long as the automobile has been with us there are many aspects ofits design and maintenance that are still not well understood by thepeople (even engineers) who have to dealt with them. (One of the reasonswhy progress in this field has been so painfully slow.) Typical of theseneglected factors is the importance of road wheel balance.

Equipment capable of properly balancing wheels is complex andexpensive--consequently not commonly available. Even then, theprinciples are often not properly understood by the mechanics who usethem so that the results are compromised.

It is no wonder that the average vehicle owner has become accustomed toan unnecessary level of roughness in the road performance of his carwhich he accepts as normal --even to the point where it is dangerous tohandling. These conditions also cause accelerated wear on tires andvehicle suspensions.

Driving along a motorway it is surprising the number of cars where oneobserves one or more wheels whose vibration is noticeably visible. Ofcourse, detrimental performance occurs far below the point of externalvisibility.

In fact, wheels should be balanced to a high degree of precision andthen rebalanced after a significant amount of tire wear has occurred,say 5 to 10 thousand miles or, of course, whenever a tire has beenremounted on a wheel for any reason, such as repair of a puncture.

Of all the components of the running gear of an automobile, wheels andtires are the least precise. Due to the manner of their manufacture theyare dimensionally relatively crude devices. The assembled combinationcan quite normally give lateral and radial run out of as much as 1/16inch (permitted by industry standards) and even 1/8 inch is notuncommon. For this and other contributory reasons intrinsic to themanufacturing methods employed, the possibility of very large massimbalances must be expected and, indeed, do occur in practice.Consequently, the wheel/tire combination must be properly balanced,laterally and radially, before mounting on the vehicle and putting it inuse.

To be properly balanced, a wheel must be in a state of equilibrium intwo respects:

1) The distribution of the mass of the wheel must be perfectlyconcentric about its axis of rotation. (Radial balance--sometimesreferred to as "static" balance.)

2) The rotational plane of the mass must be exactly perpendicular to theaxis of the rotation. (Axial balance--sometimes referred to as "dynamic"balance.)

The effect of radial imbalance is to cause the wheel to vibratevertically, or bounce as it rotates. Axial imbalance is felt as angularsteering wheel vibration--sometimes referred to on the front wheels as"shimmy".

Both of these conditions, in addition to being unpleasant for the driverand passengers, reduce the effectiveness of tire adhesion to the road.Therefore, under marginal conditions of rain, snow and ice at road speedthey can also be very dangerous.

Because of the precision with which the balancing must be carried out,it is a difficult problem to solve in a practicable, manageable way. Italso requires precision manufacture of the mounting and locatingsurfaces on the vehicle--something which the more advanced andknowledgable automobile manufacturers are now doing. For example, 0.005inch radial offset from the true center is approximately equal to a 1/2oz. weight at the wheel rim. This degree of imbalance is perceivable onmost cars. Therefore, a maximum combined error which includes thetolerances on the vehicle wheel mounting and the balancing machineshould be substantially less than 0.0025 inch.

Put another way, it must be possible to reliably detect, measure andcorrect a small imbalance in the radial plane acting at a minimal radiusof 7 inches to an accuracy of 0.03% of the wheel mass (a static torqueof about 2 inch ounces). The axial balance accuracy requirement can besimilarly stated as a 2 to 3 inch ounce static torque correction(dynamically the magnitude of this torque couple is, of course, afunction of wheel rotation velocity).

All wheel balancing methods attempt to sense the amount of massimbalance and to compensate by the installation of properly locatedincrements of weight of the required amount fastened to the wheel rim.

There are three general classes of equipment in common use for thispurpose:

1) Bubble balancers;

2) "On the vehicle" spin balancers;

3) Wheel spin balancers.

Only machines in category (3) are capable of satisfactory results.

Bubble balancers are widely used because they are inexpensive. However,they are very inaccurate and hopelessly confuse the effects of the twotypes of mass imbalance. They can, therefore, provide only a very coarsecorrection which is normally far outside the requirements for acceptableroad performance.

Machines in category (2) attempt to balance the wheels while installedon the vehicle. One of the justifications for this technique stems fromthe fact that some manufacturers do not adequately control their wheellocating dimensions. However, it can be readily shown that it istheoretically impossible to separately detect and measure the twocomponents of imbalance using this method. Nevertheless, in practice itis possible, with sufficient patience, to achieve compromise conditionswhich represent some improvement. Of course, the wheels so treated mustnot be removed and remounted in another location. This type of balancermust also be considered to be unsatisfactory.

There are a number of different types of category (3) machines.Basically, they all operate on theoretically sound principles. In oneway or the other they properly sense the two components of imbalance andprovide for their appropriate correction. However, these machines arelarge, heavy, complex and expensive. Consequently, they are out of thefinancial reach of the largest sectors of the automobile serviceindustry. Moreover, some, by their design, are susceptible to largeoperator errors so that their inherent accuracy is often not realized inpractice. But, when properly used, the better machines in this categoryrepresent the standard by which other methods should be compared.However, most such machines require that the wheel be driven atrelatively high rotational speeds to generate out of balance forceswhich are measured to provide an indication of imbalance weight andposition (e.g., see U.S. Pat. No. 3,910,121).

In my U.S. Pat. No. 3,847,025 I disclose a dynamic wheel balancingmethod and apparatus in which the wheel is mounted in a pivoted cradlemechanically coupled to a maximum displacement indicator. Themeasurements of wheel imbalance and location were all mechanical andlocating of the placement points for the weights was by trial and error.

Jackson U.S. Pat. No. 4,007,642 discloses a system using piezoelectricload cells to detect imbalance of a rotating wheel; Hale et al. U.S.Pat. No. 3,527,103 also drives a wheel at relatively high rates of speedto detect radial deviation to locate tire imperfections; Newkirk U.S.Pat. No. 1,557,268 discloses a pivoted cradle having an axle forrotating a body about an axis transverse to the cradle pivots and a longarm to an indicating scale indicates the amplitude of imbalance.

Other prior art is as follows:

Meredith U.S. Pat. No. 2,442,308

Silver U.S. Pat. No. 3,077,781

Trimble U.S. Pat. No. 3,147,624

Frank et al. U.S. Pat. No. 3,812,725

Finch et al. U.S. Pat. No. 3,991,620

Ito U.S. Pat. No. 4,011,761

Harant U.S. Pat. No. 4,149,416

Kogler et al. U.S. Pat. No. 4,173,146

THE PRESENT INVENTION

The object of the present invention is to provide a new type of wheelbalancer based on a novel principle, the execution of which exploits theadvantages of modern semiconductor electronics. The result is a machineof extremely low manufacturing cost with a number of advantageousoperating features compared to the large, complex, heavy, immovable andexpense category (3) machines disclosed in the art and presentlymarketed for this purpose. It, therefore, can bring an accurate wheelbalancing capability within the financial reach of smaller repair shops,service stations, farmers, auto hobbyists and other such groups.

According to the present invention there is provided a cradle which ispivotally mounted on a support stand for rotation about a first axis.The frame also has a precision spindle upon which the wheel to bebalanced is placed so that the wheel can be hand-rotated or spun at verylow speed about a second axis which is at an angle to the first axis sowhere the wheel is rotated at low speeds (by hand) the frame oscillatesbetween maximum and minimum amplitudes determined by the imbalance. Atransducer constituted by a coil on the stand and a magnetic member onthe frame produces a first electrical signal that is proportional tomovements of the frame about the first axis. The first electrical signalis detected and the maximum amplitude stored, and the location ofimbalance is electrically derived from this first signal. An indicatorindicates the amplitude of any axial imbalance and the location thereofwith respect to the wheel mounted in the frame. An indicator lightrotated with the wheel indicates the location on the wheel where theweight should be added.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other objects, advantages and features of the inventionwill be better understood by referring to the following detaileddescription when considered in conjunction with the accompanying drawingwherein:

FIG. 1 is a front elevational view of a wheel balancer incorporating theinvention;

FIG. 2 is a side elevational view thereof;

FIG. 3 is a sectional view on lines 3--3 of FIG. 1;

FIG. 4 is a side elevational view of the indicator assembly and spindleassembly with a wheel and tire mounted on the spindle for balancing;

FIG. 5 is a sectional view showing the electrical connection to therotating indicator;

FIG. 6 is a top view of the indicator;

FIG. 7 is a sectional view of a modification of the indicator assembly;

FIG. 8 is an enlarged elevational view of the damper assembly;

FIG. 9 is a partial side view of the damper shown in FIG. 8;

FIG. 10 is a schematic block diagram of an electrical circuitincorporating the invention;

FIG. 11A is a circuit diagram of the amplitude detector;

FIG. 11B is a circuit diagram of the phase detector connected to thetransducer shown in FIG. 11A;

FIG. 12 is a sectional (partial) side view of the wheel locating tool;

FIG. 13 is a partial top plan view of FIG. 12;

FIG. 14 shows the detail of the centering screw assembly used in thewheel locating tool; and

FIG. 15 is a graph showing the relationship between the frequency ofoscillation and phase for a resonant mechanical system with differentvalues of "Q" when driven above its natural resonant frequency (f_(o)).

DETAILED DESCRIPTION OF THE INVENTION

Referring to FIGS. 1-3, the wheel balancer includes a simple steel frame10 which suspends a welded steel cradle 11 on ball bearing pivots 13.The cradle 11 holds a precision spindle assembly 14 for accepting andvery accurately positioning the wheel with respect to both aspects ofwheel mounting (radial run out and axial normality). Its verticalorientation circumvents a common source of operator error common tovirtually all category (3) machines.

This spindle and cradle assembly 14 with a wheel positioned on it formsa very sensitive compound pendulum. The motion of the pendulum is sensedby a simple magnetic transducer 16, located at the bottom right (FIG. 1)of the frame 10.

Steel frame 10 has a pair of upright standards 10S₁, 10S₂ secured tohorizontal stabilizing subframe 10HS. Wheel cradle 11 has a pair of sidemembers 11A and 11B joined by a pair of spindle assembly support bars11C and 11D for clamping shaft 14A of spindle assembly 14 in differentheight adjustment positions. An indicator support bar 20 is removeablysupported by brackets 21 and 22 on side members 11A and 11B. Theindicator 23 is centered over the axis of spindle assembly 14 and, asdescribed later, is tethered to the wheel via the air fill valve stem ofthe tire. Transducer 16 includes a coil 16C mounted on the frame member10S₁ and a magnetic steel core element or bar 16R on wheel cradle 11 sothat relative movement between coil 16C and core element 16R induces avoltage in the coil.

Damper assembly 25 including the residual damper worker 25W (shown indetail in FIGS. 8 & 9) controls the "Q" of the mechanical assembly.

Cradle stowing assembly 26 is provided to secure the cradle 11, with awheel to be balanced carried on the spindle assembly 14, at about a 45°angle to check for radial imbalance. A stow lever 26B is pivoted at oneend on pivot 26P on standard 10S₂ and engages cradle 11 at its oppositeend. An arm 20A is connected by chain 26C to standard 10S₂ so as tolimit the downward movement of stow lever 26B. Excessive rearward cradlemovement is limited by an adjustable stop 28 (see FIG. 3).

SPINDLE ASSEMBLY

As best seen in FIG. 4, the spindle and wheel centering assemblyincludes a cone 50C rotatably and slidably centered on spindle shaft 14Abiased upwardly by spring 50S, acting between the bottom of cup shapedmember 50M and the base of cone 50C. Cup shaped member 50M is rotablysupported on spindle shaft 14 by bottom bearing element 50B received inrecesses 50R. Cup shaped member 50M has a peripheral wheel supportingflange 50PF. Cone 50C passes through wheel hole WH and accuratelycenters the wheel such that the rotary axis thereof is aligned with theaxis of spindle shaft 14A and compresses spring 50S such that asubstantial portion of the weight of the wheel W rests on peripheralflange 50PF.

Since the wheel W merely rests on peripheral flange PF, various wheelaccessories such as wire wheel covers, nut locks, etc., can be securedto the wheel W and the wheel and accessories balanced as they would beinstalled on the vehicle. Principles of this spindle arrangement can beapplied to large and small automobile tires as well as to truck tires.

INDICATOR ASSEMBLY

The position of the weight(s) to be added on the wheel is indicated by alight, such as a light emitting diode rotated synchronously with thewheel W. According to this aspect of the invention, insulated lampcarrier 60LC is rotably mounted in brass bushing assembly 60BA onindicator bar 20. The lamp 60L is at the end of lamp carrier 60C androtates adjacent (or under) scale 60SC which is carried by transparentplate 60TP on indicator bar 20. Electrical connections to lamp 60L areby connectors from the lamp 60L terminals to a ground or a commonconductor 60CG electrically connected to brass bushing 60BA and throughthe indicator arm 20 to the electrical circuit shown in FIG. 11A andFIG. 11B. Another connection at the base of lamp 60L connects toconductor 60CC and brass point contact button 60CB and connects toterminal 60T which has lead wire 60LW connected thereto leading to theelectrical circuit shown in FIG. 11A and FIG. 11B.

The lower end of shaft 60SA has towing ring 60TR secured thereto, andstiff brass wire hook 60BH extends outwardly and downwardly from towingring 60TR. As shown in FIG. 4, a tow line T has a tow ring TR at the endthereof which is slipped over the value stem V of the wheel.

An alternative way of synchronizing rotary movement of the indicatorlight 60L with wheel W is illustrated in FIG. 7. In this embodiment, thespindle shaft 14A is journaled for rotation in bearings (not shown) inframe number 11C and 11D and cup member 50M (not shown in FIG. 7) issecured to shaft 14A so as to rotate same when the wheel W is rotated.Lamp carrier 60LC is secured to the upper end of spindle shaft 14A byclamp screw 60CS in brass block 60BB to which lamp carrier 60LC issecured and in electrical connection with lamp ground conductor 60CG. Abrass point indicator disc bearing 60BPA is secured to the upper surfaceof lamp carrier 60LC' and in electrical connection with lamp conductor60CC.

Stationary transparent indicator disc 60SC' has conductive brass bearingmember 60BM secured thereto, the internal bore 60 BMI making electricalcontact with the upwardly projecting arm 60BPA, the upper pointed end60BPM forming bearing between the rotating lamp carrier 60LC' andstationary indicator disc 60SC'. A conductive connection is made toconductive brass member 60BM by rigid brass rod 60HB, which is connectedby a conductor to the electrical circuit shown in FIG. 11A and FIG. 11B.In this embodiment, the total assembly is placed on the end of shaft 14Aand clamped with clamp screw 60CS after the wheel is placed on thebalancer. Clearly, one would not go beyond the invention by using othermore elaborate means of synchronizing the position of the lamp withwheel movement and it is intended that all such synchronizing techniquesbe encompassed herein.

THE ELECTRICAL CIRCUIT

As the wheel rotates, any imbalance will cause cradle 11 to track theimbalance and oscillate about pivots 13L and 13R thereby oscillatingmagnetic element 16R in non-loading linear velocity transducer coil 16Cto thereby induce a voltage in coil 16C, the amplitude of which isproportional to the axial imbalance of the wheel and the frequency isproportional to wheel rotational velocity. Since the wheel cradle 11combination constitutes a relatively high Q resonant system (when thedamper bar is released) driven at a frequency above its natural resonantpoint, the output signal is very close to 180° out of the phase with theimbalance driving force. A simplified block diagram is shown in FIG. 10wherein channel A (shown in detail in FIG. 11A) includes circuitsamplifying and detecting the maximum or peak amplitude of the errorsignal voltage induced in the transducer coil 16C and channel B (shownin detail in FIG. 11B) includes circuitry detecting the signal phaserelative to wheel rotation position to locate the point of massimbalance. The signal from transducer 16C is amplified and peak detectedin circuit element AD and the output applied to a display, such asdisplay meter DM. The signal from transducer 16 is also amplified,filtered and the phase thereof relative to wheel position detectedinphase detector PD and the output thereof is supplied to indicatorlight means 60L, which indicates or locates the mass imbalance on thewheel itself.

Referring to FIG. 11A, signals from transducer 16C are coupled throughpotentiometer R1, to IC amplifier Ul-1, which is frequency equalized bycapacitor C6 and resistors R10 and R11 to eliminate the effect ofvariations in wheel rotational velocity. The amplitude is detected andheld by IC amplifier Ul-4, diode CR-1, and capacitor C2, IC operationalamplifier Ul-3 and the long time constant filter comprised of capacitorC3 and C4 and resistor R4, operational amplifier Ul-3 being a high inputimpedance/low output impedance for this filter. The resulting DC voltageis applied to meter driver IC operational amplifier, Ul-2, the output ofwhich is applied to display meter DM through resistor R15. Resistor R10is a zeroing adjustment for meter DM and diodes CR-3, CR-4, and IC 0amplifier U2-4, potentiometer R5, resistor R6 and diode CR-2 constitutea circuit means for limiting the meter DM indication to full scale inthe presence of excessive input signals. Large transients can occurwhenever cradle 11 is perturbed. This circuit limits the meterdeflection to the value set by R13 (normally full scale) and dumps theexcess energy from the filters. Resistor R16 couples the battery tometer DM through switch S1.

The power supply comprises 4 "D" cells providing approximately 6 volts,which can obviously alternatively be supplied from an AC supply andconverter. All operational amplifiers must operate with two equal plusand minus voltages. This is provided by operational amplifier U2-3 whichderives an accurate low impedance zero voltage. The filter comprised ofresistor R18 and capacitor C6 prevent any transient disturbances on thesupply rails from appearing on the zero rail.

Up until now there was no satisfactory way of doing anything aboutmechanical based produced noise at the input of the phase detectorbecause of the phase shift of known type of filters. The presentinvention incorporates a very novel filter, which has essentiallyzero(phase shift over the lower half of its passband. As shown in FIG.11B, the filter comprises single identical 6 DB slope sections inseries. The first such section includes the circuitry surroundingamplifiers U3-4 through U3-1. The input of amplifier U3-4 has a familiarRC-6 DB slope filter (R27, C10) at its input. The output of U3-4 thenpasses through a second filter (R39, C13) to U3-1, through an attenuatorconsisting of R-41 and R-42, an inverter U3-2 and is mixed with theoutput of U3-4 at the input of U3-3 via resistors R43 and R29. Thefilter section R-39 C-13 which is identical to R-27 C-10 has theproperty, when added out of the phase at the proper amplitude, of verynearly cancelling out the phase shift of R-27 C-10. In fact, it wouldexactly cancel this phase shift if there was not some slight amplitudeattenuation over the passband which creates a small error. The keyfactors here are the two filter section which track each other (the R-27C-10 and R-39 C-13 sections) and the amplitude of the output of R-27C-10 which is added to the output of U3-4. This coefficient of amplitudeoptimizes at the value of approximately 0.55 which is determined in thiscircuit embodiment by the gains of U3-1, U3-2, and the R-41, R-42divider. Consequently, there is a zero delay filter over the first halfof its passband. The second half of its passband is of little interestin this application since the object is to contain the total phase shiftover the operating band to the neighborhood of one degree per section.The second filter section includes the circuitry surrounding amplifiersU4-1 through U4-4.

The phase equalization network at the input of U5-2 permits it to limitat higher signal levels, consequently increasing the level to the selfbalancing zero crossing detector U5-3 and improving its accuracy as wellas doing away with the need for any offset adjustments at low levels.The phase equalization network (U5-4, U5-1, and associated circuitry) iscombined at the input of U5-2 with the normal signal and operates toprovide an increasing positive phase shift with decreasing frequency.This is exactly the opposite of what normally occurs in any kind of anRC network. R37 provides an adjustment of this phase shift from 0° toapproximately 15° at 1 Hz. U5-3 is self balancing because its DC inputsthrough R-49, R-37, and R-48 are all tied together at the output of U5-2so that any DC drift preceding this point is automatically cancelled outand U5-3 maintains essentially perfect balance for signals at alllevels.

The output from U5-3 is a square wave whose transitions are the zerocrossings of the transducer signal. A differentiator (C12, R24)generates a narrow spike at the square wave transition (zero crossing)points the negative to positive transition is passed by resistor R23;diode CR6 to U2-1. The filter circuit constituted by filter (R38, R21,C7, and C8); comparator U2-2 and diode CR5 form a gate at the input ofU2-1 to block the operation of U2-1 except for a small window in theregion of the negative to positive output from the differentiator. Thisprovides a large measure of protection of U2-1 by noise when thetransducer error signals are very small Resistor R18 couples the outputof U2-1 to the base of transistor Q, which has the collectors thereofconnected to +3 volt supply via a filter (R17, R32, and C9) and theemmiter to the indicating lamp LED 60L, which has the opposite sidethereof connected to -3 volt supply.

Exemplary integrated circuit components and values of resistors andcapacitors are set forth in the circuit diagram of FIG. 11A and FIG.11B.

THEORY AND APPLICATION OF DAMPER

A very simple friction damper has been included to control the "Q" ofthe mechanical assembly. As shown in FIGS. 8 and 9, it includes a shortaluminum damper bar 50 which bears on the left hand cradle pivot 13L. Itis loaded by spring 51 to provide about 1 lb. of force on the pivot 132.This is not a critical-parameter. Damper bar 51 is pivoted at 52 onframe member 11A, and has a series of holes 53 for adjusting theconnection of spring 51 to the bar and hence adjust the damping forceexerted by the friction between material 54 and the bearing carrier 13Cmounted cradle 11. Without appropriate damping, cradle 11 may take toolong to stabilize in phases during each test spin and its amplitude willbe unstable since a very high Q mechanical system resists being drivenat frequencies other than fo. On the other hand, too much dampingintroduces too much phase shift between cradle 11 displacement signaland the imbalance driving force.

In practice, the damper bar 50 is set to provide relatively high dampingto provide rapid settling and very stable amplitude reading. Toeliminate the phase error associated with this condition, the damper baris released for the phase measurement by pushing down on the damperlevel DL. A small amount of residual damping is retained by the dishshaped damper washer 25W to maintain a stable phase measurement while atthe same time contributing negligable error. These two damper settingsare not critical and can vary over a wide range.

ALTERNATIVE MEANS OF WHEEL LOCATION

Some automobile manufacturers do not yet use the center hole of thewheel as a means of accurately locating the wheel on the vehicle axles,but instead depend upon the wheel mounting studs to both locate andsecure the wheel. This introduces the possibility of wheel locationerror due to manufacturing tolerances on stud and hole locations.

Until now there has been no practicable way to compensate for thesetolerance errors so that. a wheel can be balanced in the same radiallocation as it: will run on the vehicle and consequently achieve a truebalance.

This alternative wheel locating tool is shown in FIGS. 13 and 14 andoperates in conjunction with some of the special features of thebalancer to solve this problem. It utilizes the center hole of the wheelaxle (not shown) as a means of identifying and retaining the truereference center about which the wheel rotates on the vehicle. Thiscenter hole is a very accurate reference since it is the axis aboutwhich the axle, and in particular its bearing surfaces were machined.

Wheel center locating tool 75 includes a clamp support bar 76 having apair of wheel rim clamp assemblies 78 (only one shown) for clamping tothe wheel rim and a centering screw assembly 77. Centering screwassembly 77 has a pilot bolt 79 passing through a bushing 79B in a largehole 80 in support bar 76, top nut 81 having a pair of pins 82 isthreadably engaged with threads 83 on pilot bolt 79. Square plates 84and 85 are welded to bar 76 and have their outer surfaces 84S and 85Smilled exactly parallel to each other. Bottom nut 86 is threadablyengaged with threads 82 on pilot bolt 79.

Rim clamp assemblies 78 include an angled bar unit 90 to which is weldeda clevis member 91 straddling the end of support bar 76. A pin 92 passesthrough one of adjustment holes 99 and a hinged adjustment screw 93,threadably engaged with one of the threaded bores 94 in support bar 76bears on angled bar unit 90 to cause it to pivot on pin 92 urging rim Rengaging ends of angled bar unit 91 to engage the rim. A pair of spacedadjustment screws 95 and 96 are carried at the ends of rim engagingmember 97 which has a pair of hard sharpened points 98 which engage therim R. Adjustment screws 98 assure that the tool 75 mounts perpendicularto the rim R.

In use, the tool 75 is installed on the wheel before the wheel bolts areloosened. This is done by holding the tool against the wheel rim R andturning the centering screw 79 until the point 79P just engages the axlecenter hole. The two adjusting screws 93 are then carefully tightened toexpand the tool against the wheel rim R to lock it in place, but in sucha way that the tool is not forced off center to any noticeable extent.The centering screw 79 is then withdrawn from the tool completely andreinstalled without the bushing 79B. The centering screw is turnedcarefully into the wheel center operating against the bottom nut 84. Thelarge diameter of this nut and the accurately machined surface of thetool will cause the screw 79 to very accurately center itself againstthe axle center hole. The top nut 81 is then spun down against the topsurface of the tool to lock the centering screw in position. In thismanner the true center about which the wheel rotates has been veryaccurately located.

One of the mounting studs and its corresponding wheel hole are marked byany suitable means such as a dab of paint and the wheel is removed fromthe vehicle.

The wheel is mounted on the balancer and the balancing is accomplishedin the same manner as previously described except that in this case thespring loaded cone is replaced by a spring loaded disc which has a smalltapered hole to accept the point 79P of the centering screw 79 which nowbecomes the radial reference in place of the wheel center hole.

After balancing, the wheel must be replaced on the vehicle in theidentical position from which it was removed as indicated by thepreviously marked stud and it may not then, or subsequently be mountedin any other location without repeating the balancing operation for thenew location.

METHOD OF OPERATION

The first operation is to correct for radial imbalance. This is done bylifting cradle 11 45° and securing it in position by the stowing lever26 on the left side of the frame as shown in FIG. 2. In this position,70% of the unbalanced concentric force fo the wheel is operative causingit to rotate to a point where this force is at the bottom. A weight ofthe proper value may then be placed at the top of the rim so that thewheel can be rotated to any position and remain stationary. Forces assmall as 10 grams (at the wheel rim) may be detected and compensated inthis manner.

Having completed this operation, radial balance has been accomplishedand the cradle 11 is restored to its original freely suspended verticalposition. The indicator bar 20 is then put in place and the line 100which tows the indicator is slipped over the valve stem 91 of the tire.

Grasping the upright at the leg top of the cradle 11 and pulling itforward to hold the cradle against the rubber stop, the wheel W is spunwith the right hand (about 1 to 2 RPS).

Releasing the cradle 11, it is now free to oscillate at an amplitude andrate which is driven by the existing axial imbalance. The amplitude ofthe imbalance is indicated by the meter 94 and its location is indicatedby the flashing LED 95 as it rotates past the circular scale 96 on theindicator bar.

The wheel is then stopped, rotated to the position where the indicatorwas flashing and a pair of weights of suitable value placed on oppositesides of the rim R. The top weight is placed opposite the zero positionon the scale and the bottom weight on the bottom side exactly 180°opposite.

The wheel W is then respun and if the meter 94 is in the green region(0-1 on a 0-10 scale) the wheel may be considered to be axiallybalanced. Axial imbalance, as small as a pair of 10 gram (or 1/4 oz.)weights, may be detected and corrected in this manner.

Because the sensitivity of this method is so high and the rotationalvelocities so low, the operation is aided by using trial magneticweights in increments of 10 grams. When the proper values are verifiedby the meter reading, normal wheel weights are substituted and hammeredin place. Thus, the entire two stage operation can be done very easilyand quickly.

The horizontal position of the wheel eliminates the need (and timeconsumed) for separately fastening the wheel in place and the taperedcone provides a very accurate and foolproof method of achieving the veryhigh standards of a concentric location. The electronics are powered b 4"D" cells. The low current drain of 20 ma (av) gives very long batterylife.

Advantages of the invention include:

1) Low Cost For the first time it brings precision wheel balancecapability within reach of every individual or organization who has aneed.

2) Portability Easily carried to any location for use. Can be stored outof the way when not in use. Bolting to the floor is eliminated. However,it is clear that the support frame 10 can be bolted to the floor if theadvantage of portability is not required.

3) Safety The lower rotating velocity (1 to 2 RPS) eliminates asignificant safety hazard inherent in present high RPM machines. Thevery low voltage, low current requirements eliminate all electricalhazards.

4) Magnetic Trial Weights Low Rotating velocities permit use of smallmagnetic trial weights to verify balance. Final wheel weights need behammered into place only once.

5) Separation of Out of Balance Forces The machine separates the axialand radial components of imbalance. This can often be useful in problemdiagnosis.

6) Compensation for Out of Tolerance Vehicle Mounting The separation ofimbalance forces and the low rotating velocity permit compensation fortolerance errors in the vehicle/ wheel mounting. This can be done bymeasuring the radial and axial errors and appropriately shimming thecone fo the back ring. Usually the errors will be much worse in theradial plane. These can also be done, but in a much easier way, by useof the alternative wheel locating tool which compensates for radialerrors without the need for explicit measurements.

7) Accurate Wheel Centering One of the biggest problems with all currentdesigns is to accurately center the wheel on the machine. A fewthousandths of an inch error introduces very large errors in the balanceprocess. The horizontal wheel position and the spring loaded coneprovide a very accurate, repeatable means of wheel location whicheliminates this operator error factor.

8) No Wheel Fastening Positive fastening of the wheel to the machine isnot required, thus saving labor and time as well as easing the use ofthe machine.

9) High Accuracy and Precision Due to factors intrinsic to themechanical and electronic principles employed, the machine is capable ofvery high levels of precision and accuracy.

10) Removable Electronics The electronics package readily disconnectsand lifts off to facilitate movement of the machine and safe keeping ofthe electronics when the machine is not in use.

11) Reliability The simplicity of the mechanics and the very low voltagestresses on the electronics enable a very high level of reliability.

12) Meter Protection A special meter protection circuit prevents damageto the meter and long recovery times due to inadvertently high inputs.

13) Balance Truck Wheels In this case, while the parts would be morerobust, Frame 11 is mounted on a stationary support stand and the truckwheel lifted onto the spindle by a fork lift, crane, etc. Slower handrotation of the wheel is possible since the mass lowers the resonantfrequency.

14) Balance Wheels with Accessories Many car wheels have accessories,such as wire wheel covers, nut locks, etc., which can cause a balancedwheel to become unbalanced after they are added. In the preferredembodiment of the present invention, since the wheel merely rests onflange 50 PF of cup member 50M, and is not secured to the apparatus, thewheel can be accurately balanced with such accessories as wire wheelcovers and nut locks on the wheel. In the case of nut locks, blind studs(shown dotted in FIG. 4) passing through the stud holes of the wheel towhich the nuts and nut locks are temporarily applied during balancing.

While there have been shown and described and pointed out thefundamental novel features of the invention as applied to the preferredembodiment, it will be understood that the various ommissions andsubstitutions and changes in the form and details of the deviceillustrated and in its operation may be those skilled in the art withoutdeparting from the spirit of the invention. It is the intention,therefore, to be limited only as indicated by the scope of the followingclaims.

I claim:
 1. A method for dynamically determining the imbalance of acombined wheel and tire for a motor vehicle, comprising the stepsof:mounting the wheel for rotation about a shaft having an axis ofrotation; statically balancing said combined wheel and tire, includingplacing said shaft at such an angel that any static unbalance will causesaid wheel and tire to rotate to indicate the static unbalance; manuallyrotating the wheel, allowing the wheel and shaft to coast and slow downto a rotational velocity in the range of from 50 to 100 rpm during asettling period; electrically measuring the forces of imbalancetransmitted by the wheel to said shaft at the termination of settlingperiod; electronically determining imbalance data related to themeasured forces; and displaying the imbalance date.
 2. The method ofclaim 1 wherein said settling period is the time it takes the measuredforces to settle as determined by the difference in signals measured onsuccessive revolutions of the shaft.